I think such a bitcoin marketplace would need some kind of trusted institution, like an escrow service. It should look something like this:

The buyer (B) buys an item on the marketplace.
He hands the cash to the escrow (let's just call it BitPal for now).
The seller ships his item to BitPal.
BitPal checks if it has the ware and the cash.
If both is at BitPal, it ships the item to the buyer and the cash to the seller.

Using this system would make transactions a lot easier, because now you just have to trust a single institution instead of hundreds of individuals.
Of course, this would kinda destroy the anonymity of Bitcoin because BitPal would have a lot of money and a lot of personal data, but it's just a thought on what is possible.

Tell me what you think!